1. Field of the Invention
The present invention relates to the frequency conversion of optical signals by nonlinear optical crystals and to laser systems that provide short-wavelength, frequency-multiplied outputs using such nonlinear optical crystals.
2. Description of the Related Art
Intense ultraviolet light sources can be used in a variety of different applications. For example, photolithography presently uses excimer lasers as light sources to take advantage of the intensity and narrow line width of excimer lasers to define features on semiconductor devices having widths of one quarter micron or smaller. Micro-machining applications use intense ultraviolet light sources, taking advantage of the flux of high-energy photons to efficiently machine fine structural details in objects. Photochemical applications can also use intense short-wavelength light sources to great advantage, because short wavelength or high-energy photons are particularly effective in driving certain types of photochemical reactions. All of these applications benefit from the use of high-intensity, short-wavelength light sources but to date the available light sources are not entirely satisfactory.
There are comparatively few light sources capable of outputting significant levels of optical output in the ultraviolet. Mercury lamps are a traditional short wavelength light source that has become increasingly inadequate for demanding or high intensity applications. Helium-cadmium and hollow cathode lasers can provide ultraviolet output, but with insufficient intensity for many applications. Excimer lasers are used in photolithography, but excimer lasers are large, expensive, and have poor beam quality. Perhaps the most significant limitation of excimer lasers for some applications, however, is the relatively low pulse rate of the laser. Typical excimer laser systems have a pulse rate on the order of one thousand Hertz, which can be disadvantageous.
Another possible ultraviolet light source is a frequency tripled or quadrupled solid state laser. The fundamental output of a solid state laser system such as Nd:YAG or Nd:YVO4 is at a wavelength of approximately 1.064 xcexcm. The frequency tripled or quadrupled output of such a solid state laser is thus in the ultraviolet, offering wavelengths of 355 nm (tripled) or 266 nm (quadrupled). Frequency multiplied laser systems are relatively complex. On the other hand, solid state laser systems have advantages compared to other intense ultraviolet light sources, making frequency multiplied solid state lasers desirable for at least some applications. The advantages of solid state lasers include excellent beam quality, relatively small size, good efficiency and the availability of high repetition rates. The reasons for certain of these advantages are discussed here to provide a better background for understanding certain aspects of the present invention.
Solid state lasers are a class of lasers that use as a gain medium a crystalline, glass or other solid material that serves as a host for an optically excitable material such as an ion of a rare earth element like neodymium. The crystalline, glass or other solid host material acts as a matrix fixing the optically excitable material in place. Examples of solid state laser systems include those that use neodymium as the excitable material within a matrix of yttrium aluminum garnet (Nd:YAG) or neodymium within a matrix of yttrium vanadate (Nd:YVO4) as the respective gain media of the solid state lasers. Pump sources for these solid state laser systems include broad band light sources such as xenon or other types of flash lamps. Often, however, the pump source for the solid state laser system is a diode laser or, more preferably, an array of diode lasers. Broad band flash lamp pumping can be inefficient because the pump light cannot be well focused into the gain region and because the spectrum of the light is so broad that light is absorbed that does not facilitate pumping of the gain medium and instead merely heats the gain medium. An alternative to broadband pump sources is the use of semiconductor laser diodes, including monolithic linear arrays of semiconductor lasers. Laser diode pump sources can be selected that output light at a wavelength closely matched to the gap between excitation levels within the gain medium. Such a matched pump source can efficiently pump the gain medium. Some solid state lasers utilize diode lasers to pump the solid state gain medium in an xe2x80x9cend-pumpedxe2x80x9d configuration in which the gain medium has its longest dimension extending along the optical axis of the laser cavity and the diode laser pump light is introduced through an end face of the gain medium. This end pumped configuration can capture a relatively high portion of the input laser diode pump light to provide an efficient laser. Side pumped configurations of solid state lasers provide laser diode array pump sources along the sides of a solid state laser. In this configuration, it is typical to direct the output of the pump diode lasers generally perpendicular to the optical axis of the solid state laser and into the gain medium. Often, side pumped configurations capture less of the pump light but use simpler optics to couple the pump light into the gain medium. Nd:YVO4 has a very high absorption coefficient at the most favorable wavelengths for efficiently pumping the Nd:YVO4 gain medium. Solid state lasers using Nd:YVO4 as a gain medium may be configured as end pumped lasers or as side pumped lasers.
FIG. 1 shows a frequency-tripled solid state laser and provides a useful illustration of several of the concepts involved in such a system. A laser diode array 10 provides pump light at a wavelength appropriate to efficiently pump the solid state gain medium and the pump light is collected and focused by a lens 12 into the gain medium 14. The gain medium in this end pumped configuration might be, for example, Nd:YAG, Nd:YVO4 or another gain medium. A coating, highly reflective at the fundamental output of the laser (1.06 xcexcm) but transmissive of the pump radiation, is deposited on one end face 16 of the gain medium 14 to serve as the high reflecting mirror for the laser cavity. Alternately, the high reflector might be a freestanding mirror transmissive at the pump wavelength but highly reflective at the fundamental wavelength of the solid state laser. Output coupler 18 defines the other end of the resonant cavity. A Q-switch 20 may be provided within the resonant cavity to provide increased levels of peak output power in pulsed mode operation.
Light 22 at a wavelength of approximately 1.06 xcexcm exits the resonant cavity through the output coupler 18 and is directed into a first nonlinear optical crystal, doubling crystal 24. In this illustration, doubling crystal 24 might be KDP (potassium dihydrogen phosphate) configured in accordance with conventional type II phase matching conditions. It may be desirable to polarize the fundamental output of the gain medium if, for example, Nd:YAG is used as the gain medium 14. On the other hand, if Nd:YVO4 is used as the gain medium 14, it may be unnecessary to provide a polarizer, since Nd:YVO4 typically provides a polarized output. The output of the doubling crystal 24 is typically chosen to include light of the fundamental frequency and light of the doubled frequency. The two output light signals are passed through a rotation element 26 that adjusts the relative polarization between the two light beams to provide the desired relationship between the polarizations of the two light beams for the tripling crystal. From the rotation element 26, the beams of fundamental and doubled light are provided into the second nonlinear optical crystal, tripling crystal 28. The tripling crystal may also be KDP operated under type II phase matching conditions.
Photons from the fundamental beam and the frequency-doubled beam are combined in the tripling crystal to provide an optical output signal at a frequency tripled from the fundamental output of the solid state gain medium 14. This frequency-tripled output may be at a wavelength of approximately 355 nm. It should be noted that the laser gain medium may be approximately one centimeter in length and the optical path length between the lens 12 and the output coupler 18 may be as small as approximately 5 to 15 cm. Consequently, these lasers may be quite compact. In addition, these frequency-tripled lasers can have good efficiency and good optical characteristics such as beam quality. As such, they are useful for a variety of applications. The illustrated optical configuration may be modified to achieve different performance characteristics. For example, the doubling crystal 24 and tripling crystal 28 can be placed within the laser cavity itself to increase the efficiency of the tripling operation.
One difficulty that has been observed for frequency-tripled, solid state lasers is a relatively low lifetime that is unacceptable for many practical applications. The shortened lifetime correlates with damage at the output face of the frequency tripling crystal and normal, frequency-tripled operation of the laser can be recovered by replacing the tripling crystal. Alternately, normal frequency-tripled laser operation can be obtained by moving the tripling crystal so that the optical path of the laser passes through a different, non-damaged portion of the tripling crystal. The strategy of moving the tripling crystal to extend the useful life of the frequency-tripled laser has been automated in a commercial laser marketed by Coherent, Inc.
Another attempt at increasing the short lifetime of frequency-tripled lasers has been to enclose sub-assemblies of the laser and to seal the sub-assemblies from the environment. One of the laser sub-assemblies that has been enclosed includes the doubling and tripling crystals, along with the optics, electronics and alignment structures necessary to use the doubling and tripling crystals within a laser system. For example, the frequency multiplying crystals 24, 28, the optical element 26, one or more heaters for the doubling and tripling crystals, as necessary, along with the wires for the heaters, and the rotational and translational alignment elements for the optical elements might all be provided within a single enclosure. Generally, the optical sub-assembly is aligned and optimized, and then the assembly is sealed within the enclosure. The optical fixtures such as rotational and translational alignment elements typically include lubricants and may not be amenable to effective cleaning techniques, such as those that include baking. The electrical lines for the heaters or other elements within the enclosure connected to the outside through wires generally must be fed through the walls of the enclosure. Seals and electronic feed-throughs that pass through the walls of the enclosure may include vacuum grease or potting compounds. This enclosure strategy has also not been successful in satisfactorily extending the normal lifetime of a frequency-tripled solid-state laser.
An aspect of the present invention provides a laser system including a solid state laser producing a fundamental laser output at a fundamental wavelength. A frequency conversion crystal is disposed along an optical path leading from the solid state laser. The frequency conversion crystal receives light and outputs light at a second wavelength different from the fundamental wavelength. A chamber encloses at least a portion of an exit face of the frequency conversion crystal within an environment hermetically sealed from an environment outside of the chamber. In a particularly preferred aspect of certain embodiments of the present invention, only elements that can be well-cleaned are positioned within the hermetically-sealed chamber.
Another aspect of the present invention provides a method for producing a laser system including providing a mounting fixture and a chamber adapted for holding the mounting fixture. The method proceeds by cleaning the mounting fixture and the chamber and cleaning at least a surface of a frequency conversion crystal to be protected by the chamber. The frequency conversion crystal, the mounting fixture and the chamber are assembled and the chamber is sealed to enclose the surface of the frequency conversion crystal to be protected by the chamber. For certain preferred embodiments, the final alignment of the conversion crystal most preferably is done after the crystal has been hermetically sealed.
Still another aspect of the present invention provides a method for replacing a frequency conversion crystal in a laser system. A laser system that uses a frequency conversion crystal to generate light of a converted wavelength is provided and the frequency conversion crystal is removed from the laser system. A new frequency conversion crystal is installed into the laser system, wherein the new frequency conversion crystal has at least one surface maintained in a hermetically sealed environment prior to and throughout installation.
Another aspect of the invention provides a method for replacing a frequency multiplying crystal in a commercial laser system, where the replacing occurs within an environment in which the commercial laser system operates.